This invention relates to the sensing of liquid levels within a vessel and, in particular, to the sensing of the level of a fluid coolant above the core in a nuclear reactor vessel.
When a nuclear reactor starts up from a cold start, the reactor vessel is completely filled with a fluid coolant such as subcooled water. During operation of the nuclear reactor, the fluid coolant is forced through the core to remove the heat generated therein. During normal operaton of a pressurized water reactor, the fluid coolant is subcooled and remains in a liquid state as it passes through the core. During abnormal operation of a pressurized water reactor due to a loss of pressure, the fluid coolant within the reactor vessel may change state to become a two-phase fluid mixture of water and steam. Due to the buoyancy of steam, the water in the volume above the core is particularly susceptible to being displaced by the two-phase fluid. The two-phase fluid in the reactor vessel head may be stagnant or turbulent two-phase flow. The two-phase fluid forms due to a decrease in pressure that results in a portion of the water flashing to steam. The difficulty of measuring the level of coolant above the core is more complex with a two-phase fluid than with a subcooled fluid. A level sensing device must be able to sense the level of liquid coolant regardless of whether the coolant is a subcooled liquid or a two-phase fluid.
Prior art instrumentation such as heated thermocouples and differential pressure sensors have provided an ambiguous and sometimes indirect indication coolant inventory above the core. An accurate indication of coolant inventory above the core is an important indication of the core cooling conditions. An accurate indication of the coolant inventory above the core would provide a sound basis for operators to insure appropriate actions are taken to prevent the coolant level from dropping below the top of the core.
Therefore, a need exists for an apparatus to provide a direct and accurate indication of the inventory of fluid coolant above the core in nuclear reactor vessels during abnormal operation as well as during normal operation. Such an apparatus would be useful in recognition of low reactor coolant levels and inadequate core cooling.